1. Field of the Invention
Embodiments of the present invention relate generally to magnetic disk drives and, more particularly, to a proximity detection method for a magnetic head and a recording medium.
2. Description of the Related Art
In a hard disk drive (HDD), the spacing between a magnetic recording head and magnetic storage disk, referred to as “head clearance,” is a critical performance parameter. Reducing head clearance during reading and writing operations can reduce bit error rate and allow accurate storage and retrieval of data that are stored on a disk at very high linear densities. Dynamic fly-height (DFH) control of read/write (R/W) heads is commonly used by modern HDDs to allow low enough fly heights for high-density storage media while maintaining sufficient head clearance over different head locations and disk drive temperatures.
For proper operation of an HDD, DFH control schemes generally require some form of calibration to determine how the fly height of an R/W head varies with stroke location, temperature, and applied DFH control signal. An important step in calibrating DFH control is determination of touchdown, i.e., when the R/W head actually makes contact with the storage medium. During normal operation such contact is avoided, but as part of calibration, touchdown provides an absolute benchmark of R/W head position relative to a disk, and is used in subsequent calibration procedures. For calibration at a given stroke location and temperature, the DFH control signal is stepped through increasing values until a portion of the R/W head begins to contact the disk. Given the touchdown control level and the actuation efficiency, i.e., the amount of fly-height change per unit of applied control signal, a DFH control algorithm can regulate the fly height accurately for an R/W head as a function of location and temperature.
A touchdown-detection algorithm is commonly used to control the HDD during HDD self-test when determining touchdown. Ideally, such a touchdown-detection algorithm can be performed by an HDD without the need for external measurement equipment. When no external equipment is needed for touchdown determination—other than the mechanical support and power supply already required for HDD self-test—the HDD test process is significantly expedited. This is because the setup and breakdown of each HDD before and after self-test can remain unchanged, thereby avoiding complications to the self-test process. In addition, a touchdown-detection algorithm should reliably determine touchdown. Declaring a touchdown power that is too low results in the R/W head flying higher than the optimal height, which can result in poor read/write quality. Declaring a touchdown power that is too high results in the R/W head flying lower than the optimal height, which can result in undesired head/disk contact and failure in the field. Further, a touchdown-detection algorithm should not require excessive head/disk contact during the measurement process for robust calibration of the R/W head position, to minimize the potential for damage to the R/W head and disk surface.
There are a number of methods known in the art for determining touchdown. One approach is to observe the gain of the read channel variable gain amplifier (VGA) as DFH power is increased, and declare touchdown when further increases in DFH power do not produce significant decreases in VGA gain. However, to detect the point at which VGA gain decreases, the DFH power must be driven past the point of initial head/disk contact, which is undesirable. Another approach involves observing acoustic output of the HDD by placing a microphone near the R/W head and declaring touchdown when a pre-determined level of acoustic output is detected. Because installation and removal of the microphone can significantly complicate setup of an HDD for the self-test process, this method is also undesirable. Another approach involves observing overall servo track misregistration (TMR) of the R/W head as DFH power is increased and declaring touchdown when the TMR exceeds a specified level. As with the VGA gain approach, accurate determination of touchdown may require overdriving the R/W head past the point of initial contact with the disk. Therefore, this approach is also undesirable for determining touchdown. In another approach, modulation of the read signal is observed from the R/W head, and touchdown is declared when a specified level of modulation is reached. Overdriving of the R/W head past initial contact with the disk may be necessary for this approach as well. In yet another approach, variance of position error signal (PES), either for total PES or specified frequency bands, is observed as a function of DFH power, and touchdown is declared when the observed variance exceeds a threshold value. While such an approach can detect touchdown for some disk drives, it has been demonstrated that for many disk drives, and even for particular tracks on a given disk drive, touchdown may not be reliably detected by this method.
In light of the above, there is a need for an improved method of touchdown detection that can be performed by an HDD during the self-test process without additional measurement equipment and does not require excessive head/disk contact.